Auxiliary power units (“APU”) are used in aircrafts to provide electrical power and compressed air to various parts of therein. When an aircraft is on the ground, its main source of electrical power comes from the APU. In particular, the APU can power the environmental control systems, air drive hydraulic pumps, and the starters for the engines. When an aircraft is in flight, the APU may provide pneumatic and/or electric power to the aircraft.
Typically, APUs are located in the aft section of the aircraft, at or near the tailcone section and include inlet and exhaust ducting that exit through an opening in the aircraft fuselage to allow sufficient air flow through to the APU. For aircrafts on which APUs operate during flight, a ram air door is typically provided to protect the APU from foreign object damage when not in use and/or during ground movement, and to maximize airflow into the APU when performance at altitude is required. Thus, when APU venting is desired, the ram air door opens, either on the ground or in flight. Typically in such configuration, the ram air door is configured to open around 45 degrees, relative to the aircraft fuselage, so that aircraft drag and entry of foreign objects into the inlet duct are minimized, while ram air recovery is optimized.
However, while the ram air door is open, noise may propagate from the APU outward from the aircraft fuselage. The noise typically travels through the inlet duct and is deflected from the interior of the ram air door to sections forward the tailcone or service locations that are located in the forward section of the aircraft. Because many aircraft sections are located forward of the APU, such as, for example, passenger doors, passenger and aircraft personnel cabins, refueling points and baggage doors, audible noise levels heard by those onboard the aircraft or those on the ground while handling baggage or performing aircraft maintenance may be increased.
Therefore, there is a need for an air inlet door that does not enhance forward propagation of inlet noise when the aircraft is on the ground. Moreover, in some cases, it is desirable for the inlet door to deflect foreign objects when the inlet door is open and while providing ram air recovery in flight. Additionally, it would be beneficial for the inlet door to cover the fuselage opening while the APU is not in operation. The present invention addresses one or more of these needs.